A speed sensing device for producing a signal corresponding to the relative angular rotation between two structural members is well known in the prior art. With the increase in electronic controls for vehicle systems, a need has been found for numerous speed sensing devices in vehicles with increasingly stringent angular motion discrimination. Contributing to this growing interest in speed sensing devices in motor vehicles is the popularity of anti-lock braking systems (ABS), the advancement of controls for automatic transmissions, and traction control. These developments have led to the incorporation of speed sensing devices into sealing packages. Such configurations incorporate a speed sensor into a radial shaft seal or into a bearing seal to keep out dirt and retain lubrication in the device.
The speed sensor reacts to a stimulus or a signal with a corresponding electrical signal. In a speed sensor device, the component which produces a stimulus corresponding to the rotating shaft speed is called a target wheel or target and the other component which reacts to this stimulus with an electrical signal is called a sensor.
The target may be either active or passive. Active targets, otherwise known as encoders, are those that produce alternating magnetic fields which are sensed and related to shaft speed, and incorporate permanent magnetization. Passive targets, known as tone wheels, are not magnetized components, but instead are usually metal rings with teeth or notches, and may be integrated within seals as well. Sensors can also be classified as active or passive, depending on whether they produce a field internally or are supplied with external power to do so. Either type of encoder can be integrated with either type of sensor. However, the combination of an active encoder with an active sensor provides advantages in size and performance. Active encoders normally require the magnetization of an elastomeric ring. This requires that the elastomer be vulcanized with ferrite powder/filaments, thus making it unsuitable for sealing. Therefore, the active encoder and seal can only be molded as one component using two-material molding techniques. Active encoders have numerous advantages, including reduction in the size of the target wheel and sensor, weight reduction, reduction in the number of parts, and integration of the components into a small seal package. The active property of the encoder allows use of Hall-effect or magneto-resistive sensors, which are smaller than other sensors used with a passive tone wheel.
The performance of speed measurement systems is constantly increasing. The use of active encoders with active sensors allows wider band widths of the sensing systems, measuring speeds from zero rpm. However, the system""s sensitivity to the air-gap, that is, the distance between the encoder and sensor, is a major consideration. This distance must be controlled by precise manufacturing and assembly, to minimize variations.
Various means exist for packaging the seal and encoder. For example, the encoder may be integrated by bonding it to the metal case of a seal, or pressing it against clamps. The sensor, due to its relative size, is usually mounted externally of the seal. It is preferable that any sensor seal design incorporate the full capability of sealing and sensing within a seal package. To accomplish this, the sealing element, encoder and sensor are all encapsulated in a metal case, with just the electric leads of the sensor protruding. Such a design would enable efficient assembly, parts reduction, and improved performance due to small air-gaps and reduced error in assembly.
As previously mentioned, manufacturing the sealing element and the encoder as one is difficult because of the unsuitable sealing properties of magnetizable elastomer. However, some efforts have been made to produce the encoder and seal simultaneously in one process (as in two-point injection molding), to bond the two to the housing after manufacture or to attach them mechanically.
Isolation of both the sensor and the encoder from their surroundings is also of high importance. Active encoders, as the source of a magnetic field, can attract contaminants and ferrite particles if subjected to internal lubrication of a bearing or a shaft. These particles originate from additives in lubricants and/or from wear particles generated due to the contact surfaces of the bearing during normal operation and can ultimately interfere with the magnetic signal by creating couplings between poles. On the other hand, in certain ABS applications, the bearing/axle may be subjected to external contaminants such as water and mud, and to other environmental disturbances such as extreme temperatures, which may affect the exposed encoder or sensor.
Some prior art designs emphasize the need for isolation of the encoder internally, while other designs identify the need to isolate the sensor and the encoder from their external environment. The need for a seal package that encloses and protects the encoder and sensor from the bearing surfaces and the surroundings is desirable.
The packaging of sensor seals creates manufacturing and assembly process challenges. The fragility of magnetic plastoferrite encoder rings is a complicating factor in manufacture and assembly, since the necessary accurate positioning and rigid clamping may increase the chances of breakage. One prior art device utilizes retaining clamps in the seal case to hold the encoder in three axes. Another prior art device relies on bonding the magnetic elastomer to the metal case prior to assembly. Inaccurate assembly might also greatly affect performance due to the above mentioned air-gap sensitivity. Aspects such as repeatability and ease of assembly have been solved by using various clamping methods or with the use of a coupling ring.
Vibration isolation has been achieved by rigid means of assembly of the encoder or by bonding the encoder to an elastomeric layer on the seal case to isolate vibration. External noise that may disturb the sensor signal is minimized by the encapsulation of the sensor within a metal case that acts as a magnetic shield, isolating the sensor from the environment and preserving magnetic energy from the active encoder.
However, these prior art devices are expensive to make and require complicated assembly procedures. Thus there remains a need for a sensor seal that is compact, inexpensive to manufacture, accurate and that does not require complicated assembly procedures.
The present invention seeks to solve these problems. The sensor seal includes a first support member having a radially extending portion and an axially extending portion and a magnetic elastomeric member on the radially extending portion. The magnetic member forms alternating, adjacent magnetic poles on the radially extending portion. A second support member is adjacent to the first support member. The second support member has a radially extending portion and an axially extending portion. A sensor member is attached to the radially extending portion of the second support member and senses the alternating, adjacent magnetic poles. An elastomeric seal member on the radially extending portion of the second support member, has a radially extending seal portion and an elastomeric body portion. The first and second support members form an internal cavity and a unitized body. The unitized body has an internal cavity that contains the sensor, the magnetic members and the elastomeric seal member. The radially inwardly extending seal portion of the seal member is contiguous with the axially extending portion of the first support member. The axially extending portion of the first support member has a low wear surface to reduce initial seal failure incidents and to enhance the life of the seal.
It is an object of the present invention to provide an encoder device that forms a unitized body with an internal cavity that contains the sensor, the magnetic member and the elastomeric seal and has a low wear surface to enhance initial seal wear-in and seal life.
It is another object of the present invention to provide a method of forming an encoder device with a magnetic member on one support member and a seal member on the other support member with one support member being folded over during the manufacturing process.
It is still another object of the present invention to provide an encoder device that is inexpensive to make, easy to install, rugged and that will seal out harmful contaminates.